Adding a cold water to tea

This morning i was making a tea. I heated up the water, till it started to boil. Usually instead of filling full cup with very hot water, i fill only half, because the rest i fill with cold water, so the tea is not so hot.
At the moment, when hot water was poured into the cup, i decided to go to check my email. And then suddenly curious question popped up into my mind.

Curious question: In which case the tea will be hotter?
  1. If I add a cold water and go for 5 minutes to check my mail
    or
  2. If I go and check my mail for 5 minutes, and when returned, add a cold water
The answer is not obvious. Because in first case difference between hot tea and room air is greater, and thus energy will flow faster away from the cup. And on the other hand, in the second case the surface between tea and air is greater, so again it means that energy flows faster away.

I did a few calculations to find the answer.



It turns out that it is very important what shape is your cup is in. And also the temperature of cold water may affect the answer. But if you have a cup made out of glass 10cm high, 8cm in diameter, 4mm tight borders, the temperature of hot tea is 70º C, cold water temperature is 15º C, room air temperature is 17º C, then if you add water before you go check email, than cup of tea will loose exactly 17.6% less energy than if you add cold water latter on.

Conclusion. In most cases, if you add water earlier to the tea, you keep your tea hotter, than if you add it latter.

Comments

Post a Comment

Popular posts from this blog

What does http-equiv stands for?

Many web developers uses meta http-equiv tag, without solid understanding what does it means. http-equiv stands for: hypertext transfer protocol equivalent and is equivalent to HTTP response header. So putting: <http-equiv="content-type" content="text/html; charset=UTF-8"> <META http-equiv="content-type" content="text/html; charset=UTF-8"> in HTML as meta TAG has the same effect as declaring HTTP response header Content-Type: text/html; charset=utf-8 That is why they are called ‘equivalent’. It is recommended to use HTTP header over HTML http-equiv meta tags. Multiple headers with the same name may be present only if the values may be concatenated.
Read more

How many water molecules are there in the glass of water?

Image
How many water molecules are in a glass of water? This glass is filled with 200 ml water. That is 0,2 l of water. First, to get started, we need to convert volume to mass. Since 1l water equals 1kg water, then our 200 ml glass is filled with 0.2 kg water. In school chemistry probably in 8th grade there was a term Mole. Mole is a unit and it measures number of molecules. One mole has 6.0221415×10 23 molecules in it. And another term is molar mass. The molar mass of an element is the atomic weight in grams per mole. In other words molar mass states - how much does one mole of particular substance weight. The molar mass for distilled water is 18 g/mol or converting to kg, it is 0,018 kg / mol. So, now lets do some calculations. First, let's calculate - how many moles of water there are in the glass. Moles = mass / molar mass = 0,2 kg / (0,018 kg/mol) = 11.11 moles And then, let's calculate how many molecules there are in the 11.11 moles Number of molecules = number of mols * m
Read more